As of the time Voldemort comes to Hogwarts to request the Defense Against the Dark Arts post how many times had Dumbledore been asked to be Minister of Magic?

As of the time Voldemort comes to Hogwarts to request the Defense Against the Dark Arts post how many times had Dumbledore been asked to be Minister of Magic?
At that time, Dumbledore told Voldemort that he was asked to be Minister three times.